The present invention is directed toward an active phased array transmitter for transmitting multiple signals and, more particularly, toward an active phased array transmitter for simultaneous transmission of multiple signals with a minimum of intermodulation distortion between signals.
Transmitters having active phased array antennas, i.e., phased array transmitters, are commonly used in the transmission of signals. Active phased array transmitters can be configured to transmit signals in highly directive beams by using multiple antenna elements each connected to an elementary transmitter power amplifier. The signals are fed through each power amplifier and are relatively phased to achieve constructive addition of radiation in the desired direction. When only a single signal is to be transmitted in a single beam, class-C single frequency power amplifiers may be efficiently utilized. The transmitted spectrum can then be determined from the phase modulation characteristics of the information modulated on the signal.
Phased array transmitters may also be configured to transmit multiple signals in multiple directed beams. In this case, combinations of the multiple signals to be transmitted are formed by a beamforming network using a set of beamforming coefficients. The signal combinations are fed to elementary transmit power amplifiers, with each power amplifier associated with a respective antenna element or subgroup of antenna elements. In this case, the power amplifiers must be linear, multiple signal power amplifiers, also known as Multiple Carrier Power Amplifiers or MCPA""s. However, a problem with this prior art configuration is that imperfect MCPA linearity causes non-linear distortion or intermodulation between the signals generating unwanted signals. These unwanted signals may lie outside the allocated frequency band, potentially interfering with other services. Even when each signal alone, and thus their linear combinations formed by a beamformer, are band limited, the non-linearly amplified combinations will exhibit out-of-band spectral components due to the non-linear distortion.
Improvements to such prior art active phased array transmitters for transmitting multiple signals in multiple beams while reducing intermodulation and improving efficiency are described in U.S. Pat. Nos. 5,548,813; 5,555,257; 5,568,088; 5,574,967; 5,594,941; 5,619,210; 5,619,503; and 5,638,024, all of which are herein incorporated by reference. The afore-listed patents are principally directed toward improving the efficiency of use of a phased array antenna and/or improving the intermodulation and efficiency performance of MCPA""s used in phased array antennas. While the afore-listed patents are not principally directed toward reducing intermodulation radiation in unauthorized frequency bands, they may provide guidance.
While a single-signal phased array for transmitting a signal in a highly directive beam may of course be provided in multiple copies in order to transmit multiple signals, the total aperture area of the plurality of single-signal antenna arrays is inefficiently used since only a fraction of the antenna elements are used to radiate each signal. For instance, if two single-signal phased array antennas were used, each would use one-half of the total antenna elements; if four single-signal phased array antennas were used, each would use only one-fourth of the total antenna elements; etc. There is thus a need for a transmitter having an active phased array antenna which employs all antenna elements to radiate each directive beam in order to obtain the full directed gain of which the total antenna aperture area is capable, while avoiding excessive inter-modulation. Further, there is a need for a transmitter having an active phased array antenna useful in a situation where no MCPA of reasonable efficiency at the current state of the art can meet the stringent limits imposed on out-of-band intermodulation radiation, thus necessitating the use of single-carrier amplifiers while still allowing multiple signals to be radiated by the antenna element array.
The present invention is directed toward overcoming one or more of the above-mentioned problems.
An active phased array transmitter is provided by disposing antenna elements on a plane or curved surface along first and second dimensions angled relative to one another forming an active phased array antenna. Antenna elements in the same row lying along the first dimension are coupled by a row-associated Butler matrix to provide a number of drive ports to which transmit power amplifiers may be connected, with successive drive ports of the Butler matrix corresponding to successive, adjacent beam directions in the plane of the first dimension in which a signal fed to the port will be radiated. Each row of antenna elements is thus capable of radiating a number of fan-shaped beams, with the fan""s wide dimension being in a plane perpendicular to the row of antenna elements, while its narrow dimension is in the plane of the antenna element row.
A first set of transmit power amplifiers is connected to corresponding drive ports of Butler matrices in different rows, a corresponding drive port being one which radiates the signal supplied to it in a fan beam in the same plane as the fan beam from a corresponding drive port of another row. The set of power amplifiers is thus arranged along the second dimension, i.e., the opposite dimension to that of the Butler matrix connections, and may thus be referred to as a column of amplifiers. Other sets of amplifiers may be connected to other columns of corresponding drive ports.
Each column of amplifiers is connected to a corresponding beamformer having an input for a signal to be radiated, a set of outputs connected to the inputs of its respective amplifier column and a phase control input to control the relative phasing of the outputs such that the fan beams radiated by different rows add constructively in only a limited angular portion within the wide dimension of the fan, thus reducing the fan beams to a spot beam which has a narrow bandwidth in both dimensions. The direction of the spot beam may be steered in the second dimension, i.e., perpendicular to the antenna element rows, by means of the phase control input, while the spot beam direction along the first dimension may be selected by routing the signal to be radiated, via a router, to an appropriate beamformer. Through use of such a transmitter, all of the elements of the phased array antenna are used to form each radiated beam, while the antenna array simultaneously radiates n different signals in different beam directions using amplifiers that only amplify a single signal at a time.
In one implementation, the above-described active phased array transmitter is borne on an orbiting satellite for communicating information to and/or between multiple user terminals located at different positions on the earth. Information to be transmitted to a given user terminal modulates a signal that is routed to the appropriate beamformer and amplifier column that is able to form a beam anywhere in a fan passing through that user terminal. The control input to the beamformer is used to select the direction within that fan corresponding to the user terminal""s exact location. Simultaneously, other columns of power amplifiers may be used to transmit to other user terminals that are located anywhere within different fans. Thus, simultaneous transmission to multiple user terminals is permitted so long as they lie in different fans. A scheduler or router groups and selects signals or information packets for simultaneous transmission such that two transmissions are never needed in the same fan at the same time. Multiple user terminals lying in the same fan are selected to be served with information at sequential times, i.e., using time-sharing or Time Division Multiple Access. The grouping may be performed with the aid of a location-related address contained with the header of data packets intended for each user terminal.
When the satellite is in a non-geostationary orbit, such as a low earth orbit, it moves rapidly relative to the earth such that the transmitted signal beams sweep across the earth at several kilometers per second. Thus, the selection of beamformer columns, via the router, and the beamformer control signals must be altered as a function of time in order to ensure that data packets transmitted to the same stationary user terminal at different times are correctly directed to the same location on the earth. This is facilitated by arranging the direction of electronic beam steering, effected with the aid of the beamformer control signals, to be along or parallel to the satellite""s ground track, while the direction of selective beam steering, by selection of the appropriate beamformer column via the router, to be in a direction at right angles to the satellite""s ground track. The beamformers may then be continuously controlled by means of a timer to compensate for orbital velocity such that, to a first order approximation, spot beams continue to be directed to fixed terrestrial locations. Due to geometrical distortions, correcting for satellite movement by routing the signal to a different beamformer to serve a particular terrestrial location is still needed, but less often than if the direction of selective beam steering was in the less favorable direction of the orbital velocity or ground track.
In another implementation, each antenna element is a dual-polarization element having two inputs and capable of radiating signals at different polarizations simultaneously, for example, horizontal and vertical or left and right hand circular polarization. One set of Butler matrices connects rows of elements using their inputs for the first polarization, and another set of Butler matrices connects rows of elements using their inputs for the second polarization. Corresponding ports of Butler matrices for the same polarization are connected to a respective column of power amplifiers and associated beamformers. In this manner, the same array of antenna elements may be used to create 2n simultaneous directive transmissions where n is the number of columns of power amplifiers provided for each polarization, while each amplifier need only amplify a single signal. The waveform of such signals may be chosen to be of constant envelope with the information modulating only the signal phase, such that efficient classC amplifiers may be used and operated at saturation without causing signal distortion.
A feature of the transmitter is the simultaneous transmission of multiple signals while employing single-signal amplifiers to reduce unwanted out-of-band radiation due to intermodulation distortion.
Another feature of the transmitter is improved efficiency of transmission using multiple, directive beams to transmit information to user terminals at different locations on the earth.
Other aspects, objects and advantages can be obtained from a study of the application, the drawings, and the appended claims.